Steam turbines typically used for power generation are comprised of multiple stages, each having fixed partitions and a plurality of turbine buckets mounted on rotatable turbine wheels. The buckets are conventionally attached to the wheels by a dovetail connection. A number of different types of dovetails may be employed. For example, a finger-type dovetail is often used to secure the buckets and rotor wheel to one another. In that type of dovetail, the outer periphery of the rotor wheel has a plurality of axially spaced circumferentially extending stepped grooves for receiving complementary fingers on each of the bucket dovetails when the buckets are stacked about the rotor wheel. Pins are typically passed through registering openings of the dovetail fingers of each of the wheel and bucket dovetails to secure the buckets to the wheel. Another type of dovetail is a tangential entry dovetail. The turbine wheel and bucket dovetails have a generally complementary pine tree configuration. Also, in gas turbines, axial entry dovetails are sometimes employed. In any event, the dovetail connections between the buckets and wheels are highly stressed and, after years of operation, tend to wear out and crack. On low pressure steam turbine rotors, cracking occurs typically as a result of stress corrosion. In high pressure steam turbine rotors, cracking typically occurs as a result of creep rupture and/or low cycle fatigue. It will be appreciated that the magnitude of the stresses in the rotor wheel are very substantial at the radial location of the wheel dovetail because of stress concentration factors developed by the dovetail geometry. That is, peak stresses are significantly higher in the wheel dovetail as compared with locations radially inwardly which have significantly lower stresses. For example, the pin openings in the finger-type dovetail, and the machined areas of the wheel defining the fingers concentrate the stresses in the dovetail area and, over time, cause cracking as a result of one or more of the aforementioned failure mechanisms.
Because of the mass and the rotational speed of a turbine, e.g., typically on the order of 3600 rpm, significant damage to the turbine, its housing and surrounds, as well as injury to turbine operators, can occur should cracks develop in the wheel dovetail sufficiently to permit one or more of the buckets to fly off the rotor wheel. Prior to the present invention, the utility operator, upon inspection of the rotor and identification of a significant crack in one or more of the turbine wheels, particularly at the dovetail connections, had essentially two choices: first, the entire rotor could be replaced and, secondly, the damaged rotor wheel could be repaired by employing a conventional weld buildup process. The first option is costly and may involve considerable costly downtime before a new rotor is available for installation. For that reason, removal of the damaged dovetail from the rotor wheel and replacement of the removed dovetail with built-up weld material has been the principal choice as the method of repairing damaged turbine wheels.
In a typical weld buildup process, the rotor is first removed from the turbine and the buckets are removed from the turbine wheel. The damaged dovetail is then removed from the wheel and weld material is applied to the rim of the wheel in multiple passes to provide a weld build up which can later be machined to provide the dovetail. The weld material can be the same as or different from the material from which the rotor wheel is made. For example, in U.S. Pat. No. 4,940,390, a TIG welding process is used to deposit a weld metal of 12 Cr material onto Ni-Cr-Mo-V. 12 Cr material is much more resistant to stress corrosion cracking than Ni-Cr-Mo-V. However, welding processes in general are prone to defects such as porosity and slag inclusions in the weld metal and it is difficult to optimize the properties of the weld material when it is being deposited on the wheel.
There are, however, specific limitations on the buildup of weld material on a wheel which render turbine rotor wheel buildups as a method of repair only marginally satisfactory. On one hand, the weld buildup material desirably should be as resistant and as strong as possible to resist stress corrosion cracking in service. On the other hand, the weld material must be weldable to the base material, i.e., the forging of the rotor. To provide such weldable material, carbon and certain other elements must be kept relatively low to render the material weldable. This results in a dovetail lower in strength as compared with what could be achieved by supplying a replacement rotor forging. Therefore weld buildups inherently limit the capability to provide optimum material for resistance to stress corrosion cracking, creep rupture and cycle fatigue. The resulting weld buildup typically sacrifices tensile and yield strength to accommodate the need for a material weldable to the base material. This lower strength, together with the tendency for weld defects to grow during operation of the turbine, can limit the life expectancy of the repair to well below that of a replacement rotor forging.
Stress relief is also an important consideration in employing weld buildups for rotor wheel repair. Typically, the weld buildup is applied to the rotor, while the rotor axis lies in a horizontal position. However, to stress-relieve the rotor by application of heat, conventional methodology provides for hanging the rotor vertically, i.e., the rotor axis lies vertically. It was believed that the application of heat for stress relief purposes must be applied while the rotor is vertical to achieve uniformity of applied heat and uniform stress relief about the rotor. This involves substantial handling of the rotor with attendant risk of damage to the rotor.
Accordingly, there has developed a need for a repaired turbine rotor wheel dovetail wherein the material of the dovetails has the same as or increased resistance to failure mechanisms, such as stress corrosion cracking, creep rupture and cycle fatigue, as well as improved methods for repairing the dovetails of turbine rotor wheels.